The present invention relates in general to a device for correcting chromatic and other types of aberration, and more particularly, to a dichroic Mangin mirror operative to achromatize a large spectrum and correct aberration.
When a light beam composed of multiple wavelengths propagates through a medium, the index of refraction is wavelength dependent. The wavelength dependence is known as dispersion. A well-known dispersion example is a glass prism that disperses an incident beam of white light into a rainbow color. Many optical devices or elements, such as photographic lenses that comprise various dispersive, dielectric glasses, do not refract all constituent wavelengths of an incident light beam at the same angle. This causes the departure of a perfect imaging for a multi-wavelength light beam. For a color light beam, the departure of perfect imaging due to dispersion is referred as a chromatic aberration. Two types of chromatic aberration are commonly seen, including the longitudinal and transverse chromatic aberration. The longitudinal chromatic aberration is the inability of a lens to focus different colors at the same focal point. For example, as shown in FIG. 1, for a subject point on the optical axis the foci of the various colors are also on the optical axis, but displaced in the longitudinal direction (i.e. along the axis). The transverse chromatic aberration, also referred as lateral color, often occurs when the light beam is obliquely incident on the lens as shown in FIG. 2.
In addition to the chromatic aberration, monochromatic aberration, including spherical aberration, coma, oblique astigmatism, curvature of field, and distortion also affect imaging of an incident light beam. Many optical designs, including lens doublets or triplets composed of lenses with compensation characteristics, have been used to eliminate the aberration. However, most of these optical designs, though effective to resolve one of the aberrations, will inevitably cause other adverse optical effects or the design may be ineffective for removing more than one aberration.